Compute the Capacitor Values for a Crystal Oscillator

The capacitors C1 and C2 make up the rest of the basic oscillator circuit when combined with the inverting amplifier and the crystal or resonator. The optimum load capacitance (CL) for a given crystal is specified by the crystal manufacturer. CL can be calculated using the Equation 1 from Application note AN849 - Basic PICmicro Oscillator Design.

Equation 1:
CL = (C1 * C2) / (C1 + C2) + Cstray

Example: If CL = 12.5 pF (optimum load capacitance provided by the crystal manufacturer) and assuming that Cstray = 2 pF and C1 = C2;
using Equation 1:

C1 = 2(12.5 pF-2 pF) C1 = 2(10.5 pF) = 21 pF

In this example, do not use a loading capacitor below 21 pF. We recommend using the next standard 22 pF ceramic capacitor.

© 2024 Microchip Technology, Inc.
Notice: ARM and Cortex are the registered trademarks of ARM Limited in the EU and other countries.
Information contained on this site regarding device applications and the like is provided only for your convenience and may be superseded by updates. It is your responsibility to ensure that your application meets with your specifications. MICROCHIP MAKES NO REPRESENTATIONS OR WARRANTIES OF ANY KIND WHETHER EXPRESS OR IMPLIED, WRITTEN OR ORAL, STATUTORY OR OTHERWISE, RELATED TO THE INFORMATION, INCLUDING BUT NOT LIMITED TO ITS CONDITION, QUALITY, PERFORMANCE, MERCHANTABILITY OR FITNESS FOR PURPOSE. Microchip disclaims all liability arising from this information and its use. Use of Microchip devices in life support and/or safety applications is entirely at the buyer's risk, and the buyer agrees to defend, indemnify and hold harmless Microchip from any and all damages, claims, suits, or expenses resulting from such use. No licenses are conveyed, implicitly or otherwise, under any Microchip intellectual property rights.